1. Field of the Invention
The present invention relates to expansion devices for refrigeration systems, and more particularly to a low cost, self-cleaning expansion device having a high efficiency.
2. Description of the Prior Art
In a refrigeration system, the liquid refrigerant from the condensing unit is at a relatively high pressure. This pressure must be reduced in the evaporator such that the refrigerant will evaporate at a low temperature. Thus, an expansion device must be provided between the liquid line and the input to the evaporator. There are a number of requirements for this device. The expansion device must, in effect, meter the liquid into the evaporator in an amount equal to that required to provide the desired refrigeration effect and which will prevent liquid refrigerant from entering the suction line to the compressor. In addition, it is desirable, when the compressor is off, that pressure equalize between the liquid side and the vapor side to minimize the startup load. The most efficient devices are expansion valves which may be operated from pressure or temperature or both. However, due to the complexity and cost of expansion valves, it is most common for small refrigeration systems such as room air conditioners and refrigerators to utilize a capillary tube control. This device is simply a length of small inside diameter tubing which may be from 18" to 12' long which throttles the movement of liquid refrigerant into the evaporator. The principle of operation involves the capillary tube's resistance to fluid flow. The pressure therefore drops as the liquid moves through the tube to the point where it begins to evaporate. The evaporation causes a sudden pressure and temperature drop as the refrigerant enters the evaporator. The capillary tube has the advantage of permitting equalization of pressure when the compressor is not running. Although the capillary tube is a low cost device, it requires a fine filter or filter-dryer at its inlet since any moisture or dirt which may flow into the capillary tube will cause it to become plugged. Similarly, a problem occurs if the capillary becomes plugged with ice or wax.
A more serious disadvantage of the capillary tube is the energy required to move the refrigerant through the tube. This is of course reflected into an increase of primary power.
Thus, there is a need for a simple expansion device which will have the advantages of the capillary tube of low cost, and pressure equalization but without the disadvantages of moisture, dirt and ice causing plugging, and of power loss through the tube.